1. Field of the Invention
The present invention relates to an optical system of an optical pickup apparatus, and more particularly to an optical system for correcting spherical aberration.
2. Related Background Art
In recent years, in order to realize a high recording density of an optical disk apparatus, techniques for shortening a wavelength of a light source and increasing the NA of an objective lens have been actively researched. Commercialization of an apparatus using a 405 nm semiconductor laser and an objective lens having the NA of 0.85 is started.
When the short-wavelength light source and the high-NA objective lens are employed, there are the following fundamental problems:
(1) The optical disk apparatus is easily affected by the tilt of the disk;
(2) The optical disk apparatus is easily affected by a thickness error of a transparent substrate; and
(3) The optical disk apparatus is easily affected by wavelength hop of the light source.
Among those, the problem (1) is solved by thinning the transparent substrate. For example, a thickness of the transparent substrate is set to about 100 μm.
In order to solve the problems (2) and (3), an attempt has been made to devise an optical system. For example, in order to prevent the wavelength hop of the light source in (3), there has been made an attempt to provide an additional chromatic aberration correction lens for producing chromatic aberration to cancel chromatic aberration caused in the objective lens.
With respect to the problem (2), there have been studied the following methods of canceling spherical aberration caused when the transparent substrate has a thickness error, for example, (a) a method of providing an additional beam expander composed of lenses and changing an interval between the lenses to produce spherical aberration for canceling the spherical aberration caused when the transparent substrate has a thickness error, and (b) a method of shifting a position of a collimator lens in an optical axis direction to produce spherical aberration for canceling the spherical aberration caused when the transparent substrate has a thickness error.
The above-mentioned techniques have been disclosed by, for example, Japanese Patent Application Laid-Open No. 2002-236252.
FIG. 4A shows the case where the beam expander is used. To explain it briefly, as shown in FIG. 4A, an expander 33 composed of a negative power lens 31 and a positive power lens 32 is provided on an optical path of a parallel light flux on the light incident side of the objective lens 35. An interval between the lens 31 and the lens 32 is changed according to a thickness error of a transparent substrate 36 of an optical disk to produce the spherical aberration.
FIG. 4B shows the case where the position of the collimator lens is shifted. A system as shown in FIG. 4B is used and a collimator lens 34 is moved along an optical axis according to the thickness error of the transparent substrate 36 of the optical disk to produce the spherical aberration.
However, the above-mentioned conventional cases have the following problems. That is, when an additional beam expander is employed, a size of an optical pickup apparatus is likely to increase because an additional optical element is provided.
When the position of the collimator lens is shifted, it is necessary to move the collimator lens over a given movable distance, which is about 10 times a focal depth thereof. For example, when the NA of the collimator lens is about 0.1 and the thickness error of the transparent substrate is nearly equal to ±7 μm, it is necessary to set a movable range of about ±1 mm. As a result, the size of the optical pickup apparatus is likely to increase.